jahresbericht 2007 - Institut für Kernchemie - Johannes Gutenberg ...

Status of the TASCA Commissioning Program *
M. Schädel 1,# , D. Ackermann 1 , W. Brüchle 1 , Ch.E. Düllmann 1 , J. Dvorak 2 , K. Eberhardt 3 , J. Even 3 ,
A. Gorshkov 2 , R. Gräger 2 , K.E. Gregorich 4 , F.P. Heßberger 1 , A. Hübner 1 , E. Jäger 1 , J. Khuyagbaatar
1 , B. Kindler 1 , J.V. Kratz 3 , D. Liebe 3 , B. Lommel 1 , J.P. Omtvedt 5 , K. Opel 5 , A. Sabelnikov 5 , F.
Samadani 5 , B. Schausten 1 , R. Schuber 2 , E. Schimpf 1 , A. Semchenkov 1,2,5 , J. Steiner 1 , J. Szerypo 6 , A.
Türler 2 , and A. Yakushev 2 for the TASCA Collaboration
1 GSI, Darmstadt, Germany; 2 Technical University München, Garching, Germany; 3 University of Mainz, Mainz, Germany;
4 LBNL, Berkeley, CA, U.S.A.; 5 University of Oslo, Oslo, Norway; 6 LMU München, Garching, Germany
The TransActinide Separator and Chemistry Apparatus,
TASCA, project [1] is focusing on the separation and
investigation of neutron-rich transactinide nuclides produced
in actinide target based reactions. The envisioned
research program includes both chemical investigations of
transactinide or superheavy elements after preseparation
with the gas-filled separator and physics motivated nuclear
structure and nuclear reaction studies.
The central device of TASCA is a gas-filled separator in
a DQQ configuration. It can be operated in the "High
Transmission Mode" (HTM, DQ h Q v ) and in the "Small
Image Mode" (SIM, DQ v Q h ); see Refs. [1-4] for more
details. The separator was installed at the UNILAC beam
line X8 and, after having all crucial parts of the control
system [5] running, an extensive commissioning program
was carried out in 2007. This report briefly summarizes
the nuclear reactions applied and the most important parameters
studied. A few examples are discussed in a very
exemplary way. In addition, recent target developments
and the progress in the coupling of chemistry set-ups will
be outlined. The first chemical study behind TASCA is
described in a separate contribution [6].
All nuclear reactions applied are listed in Table 1 together
with the mode of TASCA operation (HTM=H,
SIM=S) and the separator gas. Also indicated are experiments
aimed to test or apply a recoil transfer chamber
(RTC) in addition to measurements performed with a focal
plane detector (FPD). As the standard FPD we used a
(8x3.6) cm 2 large position-sensitive 16-strip silicon detector.
Some experiments were devoted to test prototype
double-sided silicon strip detectors (DSSSD) which are
planned to be used in future experiments with superheavy
elements (SHE).
To understand TASCA as a separator and to build up a
solid data base providing good predictive power concerning
separator operation for future SHE experiments, we
investigated the following most important parameters: (i)
the magnetic rigidity of reaction products between Z=76,
Os, and Z=102, No, produced at different recoil velocities,
and the corresponding best settings of the dipole magnet,
(ii) the quadrupole focusing, which is especially relevant
for the SIM, (iii) the target thickness dependence of the
separator transmission - strongly depending on the asymmetry
of the nuclear reaction -, and (iv) the optimum gas
pressure with respect to focusing and to transmission -
___________________________________________
* Work supported by BMBF (06MT247I, 06MT248, 06MZ223I) and
GSI-F&E (MT/TÜR, MZJVKR)
#
m.schaedel@gsi.de
being quite different in the HTM and in the SIM. The
analysis of a huge amount of data from these experiments
is in progress, and it is important to realize that most of
the above mentioned parameters influence each other.
Table 1: Nuclear reactions applied in TASCA commissioning
experiments; see text for details.
Beam Target Product Mode Gas RTC
22 Ne
nat Ta
198m-199 Bi H + S He
179 Au
215 Ac H + S He
238 U
255 No H + S He
30 Si no
30 Si H + S Vac
181 Ta
205-206 Fr H He
40 Ar
nat Ce
173,175 Os H He yes
144 Sm
180-182 Hg H + S He yes
nat Gd,
194-196 Pb, H + S He yes
48 Ca
54 Cr
152 Gd
nat Lu
208 Pb
232
Th,
238 U
144 Sm
206 Pb
208 Pb
nat Gd
188 Pb
210 Ac H + S He, N 2
245 Fm H + S He yes
targettest, H He
background
188 Pb H + S He
252 No H + S He
254 No H He, H 2
209-210 Ra H + S
Always as a first step, the best dipole setting was found
in HTM by centring the product distribution with a typical
width of ≈ 6 cm on the FPD. A magnetic rigidity range
from 1.5 to 2.2 Tm was covered in those experiments. The
quadrupole focusing was found to be insensitive to small
quadrupole current changes in the HTM while it reacts
very sensitively in the SIM. Optimized SIM settings were
determined to obtain maximum rates and narrow distributions
of ≈ 1.5 cm FWHM.
The target thickness dependence of the transmission
was extensively studied in the reactions 22 Ne + 197 Au (55,
130, 255, 580 µg/cm 2 ) and 40 Ar + 144 Sm (75, 190, 380,
930 µg/cm 2 ) in both modes. A comparison of these data
with model calculations [7] will allow selecting an optimum
target thickness with the highest product rate for all
the envisioned nuclear reactions.
Many experiments were devoted to find the optimum
He pressure and to determine the response to pressure
changes. For this we checked the spatial distribution and
the total rate of the products in the FPD. While a pressure
of about 1 mbar is generally best in the HTM, a signifi-
- A10.1 -